Qubit and Readout Overlap Optimization via Dipole Orientation

The present disclosure generally relates to quantum computation systems.

Quantum computation exploits quantum phenomena for information processing and communication. Various models of quantum computation exist, and the most popular models include the concepts of qubits and quantum gates. A qubit is a generalization of a bit that has two possible states, but can be in a quantum superposition of both states. A quantum gate describes the transformation that one or more qubits will experience after the gate is applied on them, given their initial state. Various quantum phenomena, such as superposition and entanglement, do not have analogs in the world of classical computing and therefore may involve special structures, techniques, and materials.

The state of a qubit can be read out by coupling the qubit dispersively to a resonator circuit. As the different qubit states shift the resonance frequency differently, the frequency change can be detected by measuring the phase of a microwave pulse reflected on (or transmitted through) the resonator.

Some embodiments of the disclosure provide a qubit device with reduced size. The device includes a first set of circuit components implementing a qubit and a second set of circuit components implementing a readout resonator for reading out a state of the qubit. A first footprint of the first set of circuit components overlaps a second footprint of the second set of circuit components. The orientation of the first footprint relative to the second footprint is determined based on a first electric dipole of the first set of circuit components and a second electric dipole of the second set of circuit components.

The qubit device may include a substrate and an interposer, where the first footprint is on a first surface of the substrate and the second footprint is on a first surface of the interposer. The qubit device may include a substrate, where the first footprint is on a first surface of the substrate and the second footprint is on a second surface of the substrate in a flip-chip configuration.

In some embodiments, the first and second footprints are oriented to minimize coupling effect between the qubit and the readout resonator. The first electric dipole of the qubit and the second electric dipole of the readout resonator are orthogonal.

In some embodiments, the first and second footprints are oriented to enhance a coupling effect between the qubit and the readout resonator. The first electric dipole and the second electric dipole may be parallel. The qubit and the readout resonator may not be physically in contact. The coupling effect between the qubit and the readout resonator is sufficient to provide a state of the qubit to the readout resonator.

3 4 In some embodiments, the readout resonator operates in a higher order mode having more than two charged sections, while the Josephson Junction (JJ) implementing the qubit includes more than two (e.g.,or) superconductor pads that are configured to couple to a particular resonator mode based on a geometry between the qubit and the readout resonator.

By controlling the orientation of the readout resonator relative to the qubit, the coupling effect between the qubit and the readout resonator can be suppressed or enhanced. In addition, by overlapping the readout resonator and the qubit in a qubit device, higher component density and smaller chip size can be realized to achieve better scalability.

The preceding Summary is intended to serve as a brief introduction to some embodiments of the disclosure. It is not meant to be an introduction or overview of all inventive subject matter disclosed in this document. The Detailed Description that follows and the Drawings that are referred to in the Detailed Description will further describe the embodiments described in the Summary as well as other embodiments. Accordingly, to understand all the embodiments described by this document, a Summary, Detailed Description and the Drawings are provided. Moreover, the claimed subject matter is not to be limited by the illustrative details in the Summary, Detailed Description, and the Drawings, but rather is to be defined by the appended claims, because the claimed subject matter can be embodied in other specific forms without departing from the spirit of the subject matter.

In the following detailed description, numerous specific details are set forth by way of examples in order to provide a thorough understanding of the relevant teachings. However, it should be apparent that the present teachings may be practiced without such details. In other instances, well-known methods, procedures, components, and/or circuitry have been described at a relatively high-level, without detail, in order to avoid unnecessarily obscuring aspects of the present teachings.

For a quantum computing system based on qubits, one limitation to scalability is the density of components that are used to implement a qubit in a qubit chip or qubit device. Currently, most components of a qubit device are kept shielded from each other (with bumps, vias or large spacing) to avoid unwanted interactions among the components. This requirement for shielding components from each other places a limit on increased chip density. With the recent push to increase the number of qubits implemented on each qubit device, the minimization of the on-chip footprint of a qubit and its connected supporting circuitry is becoming increasingly salient. Currently, some of the qubit’s supporting circuitry, such as the readout resonator, is placed on a separate resonator chip (e.g., on an interposer) away from the qubit chip. This is due to concerns of undesirable coupling between the readout resonator and the qubit.

Some embodiments of the disclosure provide a method for decreasing the footprint of a qubit in a qubit device. The qubit device implements a qubit circuit and a readout resonator circuit. The qubit circuit and the readout resonator circuit are on different planes, with one plane above the other. In some embodiments, to mitigate the concern of undesirable coupling, the electric dipoles of the qubit circuit and the readout resonator circuit are set to be orthogonal, resulting in no change to the total coupling between the two circuit structures. The qubit-resonator interactions are enhanced or suppressed by inter-plane coupling with accurate control provided by the exact orientation of the electric dipole. In some embodiments, a readout resonator and a transmon implementing a qubit overlap in a flip-chip design, without impact to the readout chip. In some embodiments, the readout resonator circuit may be placed on an interposer above the qubit circuit.

1 4 FIGS.- 1 FIG. 3 100 100 110 120 110 120 130 illustrate a qubit device design in which the readout resonator circuit is placed above the qubit circuit, with electric dipoles of the two circuit structures being orthogonal.shows aD representation of a qubit devicehaving a qubit device design that is consistent with an illustrative embodiment. The qubit device designincludes a transmon qubit circuitand a readout resonator circuit. The transmon qubit circuitoverlaps the readout resonator circuitin different planes or layers. The circuit components of the different planes or layers are interconnected by a via.

2 FIG. 3 FIG. 2 100 110 120 100 310 110 320 120 130 110 310 120 320 shows a correspondingD representation of the qubit device, consistent with an exemplary embodiment. In the figure, the circuit components of the transmon qubit circuitare shown in solid lines while the circuit components of the readout resonator circuitare shown in dotted lines.shows different layers of the qubit device, in which a first layerincludes the circuit components of the transmon qubit circuitand a second layerincludes the circuit components of the readout resonator. The via structureconnects the transmon qubit circuitfrom the first layerto the readout resonatorin the second layer.

4 FIG. 110 120 100 410 420 410 420 conceptually illustrates the electric dipoles of the transmon and the readout resonator. As illustrated, the orientations of the transmon qubit circuitand of the readout resonatorare configured such that during operations of the qubit device, the electric charges of the two circuit structures form dipoles(of the transmon) and(of the readout resonator). In this example, the two dipolesandare orthogonal.

5 FIG. 100 100 500 110 500 500 130 As mentioned, in some embodiments, the readout resonator and the transmon qubit overlap in a flip-chip configuration without impact to the readout chip, such that the footprint of the transmon qubit is on a first surface of a substrate and the footprint of the readout resonator is on a second surface of the substrate.illustrates a cross section views of the qubit devicethat is implemented on as a flip-chip design, consistent with an illustrative embodiment. As illustrated, the qubit deviceis built on a substrate. The circuit components of the qubit transmonpopulates the top surface of the substrate, while the circuit components of the readout resonator populate the bottom surface of the substrate. The viainterconnects the two circuit structures at the surfaces of the substrate.

6 FIG. 100 100 610 110 610 120 620 130 620 610 In some embodiments, the readout resonator and the transmon qubit overlap in a substrate-interposer configuration, such that the footprint of the transmon qubit is on a surface of a substrate and the footprint of the readout resonator is on a surface of an interposer above the substrate.illustrates a cross section view of the qubit devicein which the readout resonator is implemented on an interposer, consistent with an illustrative embodiment. As illustrated, the qubit deviceis built on a substratehaving an interposer mounted above through bumps. The circuit components of the qubit transmonpopulate the top surface of the substrate, while the circuit components of the readout resonatorpopulate a surface of an interposer. The viais implemented by one of the bumps that mount the interposerto the substrate.

120 110 Though the readout resonatoris placed immediately above the transmon qubit(in order to reduce chip area), it is empirically determined that, by having the electric dipoles of the two structures to be orthogonal, the coupling is similar to when the readout resonator circuit and the qubit transmon circuit are placed far apart or isolated from each other.

7 FIG. 700 710 720 710 100 700 illustrates a qubit devicein which a qubit transmonis isolated from a readout resonator. The qubit transmonand the readout resonator are separated to minimize coupling effect. Simulations show that, when the dipoles of the two structures are orthogonal to each other, the qubit devicehaving the qubit transmon overlapping the readout resonator can have coupling effect nearly as low as when the qubit transmon and the readout resonator are isolated from each other (e.g., the qubit device).

1 4 FIGS.- 8 FIG. 100 800 810 820 810 820 800 815 825 810 820 show a qubit devicein which the qubit transmon and the readout resonator are positioned to substantially minimize coupling (e.g., to reduce coupling from a maximum coupling condition.) In some embodiments, a qubit device may have the qubit transmon and the readout resonator positioned and oriented to enhance coupling, specifically by aligning the electric dipole of the two structures.illustrates a qubit devicein which a qubit transmonand a readout resonatorare positioned and oriented to enhance coupling. As illustrated, the orientations of the transmon qubit circuitand of the readout resonatorare configured such that during operations of the qubit device, the electric charges of the two circuit structures form dipolesandthat are parallel or aligned in a same direction, thereby enhance the coupling between the qubit circuitand the readout resonator.

1 8 FIGS.- In the examples of, the interconnect between the qubit and the resonator relies on the capacitance of the gap (between the qubit and the readout resonator) to couple the qubit and the resonator, and by using the optimal dipole orientation to keep sufficient coupling between the qubit transmon and the readout resonator. The coupling effect between the qubit and the readout resonator is sufficient to provide a state of the qubit to the readout resonator.

In some embodiments, the qubit device is configured to use higher order modes of the resonator and to only have additive field coupling at those modes (while having subtractive field at other modes). This may allow for multiple qubits on the same readout resonator, but at different modes/frequencies.

9 FIGS.A-B 9 FIG.A 900 910 920 910 911 912 920 910 920 911 912 910 920 conceptually illustrate an example layout of a qubit circuit that uses higher order modes of the readout resonator to reduce or enhance coupling, consistent with an illustrative embodiment. The figure illustrates a qubit devicehaving a qubit transmonand a readout resonator. The qubit transmonis implemented by a Josephson Junction having two superconductor padsand.shows the readout resonatoroperating in a higher order mode that enhances coupling effect between the qubit and the resonator due to the electric fields from the superconductor padsbeing additive to the electric fields of the resonator. Specifically, each superconductor pad (and) of the qubitis positioned near a charged section of the resonatorthat has an opposite charge.

9 FIG.B 920 910 920 911 912 910 920 shows the readout resonatoroperating in an alternative mode that suppresses the coupling effect between the qubit and the resonator due to the electric fields from the superconductor padsbeing subtractive to the electric fields of the resonator. Specifically, each superconductor pad or pad (and) of the qubitis positioned near a charged section of the resonatorthat has a same charge.

In some embodiments, an alternative qubit design is used to enhance the coupling effect between the qubit and the resonator. Specifically, the readout resonator operates in a higher order mode and the qubit transmon is implemented by a Josephson Junction (JJ) having more than two superconductor pads. The superconducting pads of the JJ are configured to couple to a particular resonator mode based on a geometry between the qubit and the readout resonator. For example, each superconductor pad may be positioned to overlap a charged section of the readout resonator carrying an opposite charge from the superconductor pad to enhance a coupling effect between the qubit and the readout resonator. The superconductor pads are also positioned such that, for other modes of the readout resonator, there is no effective coupling.

10 FIG. 1000 1010 1020 1010 1011 1014 1021 1024 illustrates an alternative qubit design that uses a higher harmonic mode of the resonator to enhance the coupling effect, consistent with an illustrative embodiment. In the example, the qubit has four superconductor pads that are positioned to match the harmonic mode of the resonator. As illustrated, a qubit devicehas a qubitthat overlaps its readout resonator. The qubitis implemented by Josephson Junction having four superconductor pads-. The resonator is operating in a higher order mode such that different charged sections-of the resonator have different electrical charges with different electric field directions.

1010 1020 1011 1012 1013 1014 1021 1023 1022 1024 1011 1021 1012 1023 1013 1022 1014 1024 1010 1020 The different superconductor pads of the qubitare positioned to overlap the different charged sections of the resonator. At the instance in time illustrated, the superconductor padsandof the qubit have positive charges while the superconductor padsandhave negative charges. The resonator sectionandhave negative charges while the resonator sectionsandhave positive charges. As illustrated, the superconductor padoverlaps resonator section, the superconductor padoverlaps resonator section, the superconductor padoverlaps resonator section, and the superconductor padoverlaps resonator section. The additive fields at the different overlapping sections and superconductor pads enhance the coupling effect of the qubitand the resonator.

The descriptions of the various embodiments of the present teachings have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

While the foregoing has described what are considered to be the best state and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that the teachings may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all applications, modifications and variations that fall within the true scope of the present teachings.

The components, steps, features, objects, benefits and advantages that have been discussed herein are merely illustrative. None of them, nor the discussions relating to them, are intended to limit the scope of protection. While various advantages have been discussed herein, it will be understood that not all embodiments necessarily include all advantages. Unless otherwise stated, all measurements, values, ratings, positions, magnitudes, sizes, and other specifications that are set forth in this specification, including in the claims that follow, are approximate, not exact. They are intended to have a reasonable range that is consistent with the functions to which they relate and with what is customary in the art to which they pertain.

Numerous other embodiments are also contemplated. These include embodiments that have fewer, additional, and/or different components, steps, features, objects, benefits and advantages. These also include embodiments in which the components and/or steps are arranged and/or ordered differently.

While the foregoing has been described in conjunction with exemplary embodiments, it is understood that the term “exemplary” is merely meant as an example, rather than the best or optimal. Except as stated immediately above, nothing that has been stated or illustrated is intended or should be interpreted to cause a dedication of any component, step, feature, object, benefit, advantage, or equivalent to the public, regardless of whether it is or is not recited in the claims.

It will be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. Relational terms such as first and second and the like may be used solely to distinguish one entity or action from another without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “a” or “an” does not, without further constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.